Move noSuchMethod hook from UnknownElement to Element

lib/dom/templates/html/impl/impl_UnknownElement.darttemplate

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// Temporary dispatch hook to support WebComponents.
-Function dynamicUnknownElementDispatcher;
-
-class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
-$!MEMBERS
-
-  // Hooks to support custom WebComponents.
-  var xtag;
-
-$if DARTIUM
-  noSuchMethod(String name, List args) {
-    if (dynamicUnknownElementDispatcher == null) {
-      throw new NoSuchMethodException(this, name, args);
-    } else {
-      return dynamicUnknownElementDispatcher(this, name, args);
-    }
-  }
-$else
-  // TODO(vsm): Implement noSuchMethod or similar for dart2js.
-$endif
-}